This invention relates to an infrared-transparent window structure, and, more particularly, to such a window structure that is, in part, electrically conductive.
Sensors operating in the infrared wavelength bands are used in civilian and military applications. Such sensors are usually rather fragile in construction and susceptible to external damage. They are therefore usually placed behind a protective window that is transparent to the radiation being sensed but protects the sensor from damage due to hostile physical and chemical environmental effects, impacts, and the like.
A mechanically protective window is sufficient for some applications, but in other cases, such as many military applications, the window must also protect against penetration of radio frequency energy to the sensor. Various techniques are known for attenuating incident radio frequency energy while allowing the transmission of the infrared radiation of interest. These techniques operate by a variety of reflection and absorption mechanisms, but typically require that the window have at least some degree of electrically conductivity. For example, it is known to place a metallic grid pattern formed of narrow, electrically conductive metallic stripes onto the surface of the window, which grid pattern reflects the incident radio frequency energy away from the window while passing most of the infrared radiation.
While operable, grid structures in the window have the disadvantage that they may in some cases adversely affect the sensor performance by increasing stray light reaching the sensor, reducing optical transmission of the window, introducing ghost images, and distorting the images reaching the sensor. There is therefore a need for an improved approach to infrared-transparent windows that have the necessary electrical conductivity required to protect the sensor against the intrusion of electromagnetic energy and to reflect electromagnetic energy. Such a window must have high transmittance and desirably has modest cost The present invention fulfills this need, and further provides related advantages.